The proposed studies will explore the ontogeny of fetal central osmotic-dipsogenic mechanisms to develop an understanding of normal and abnormal fluid homeostatic responses. The investigators hypothesize: a) osmotic-dipsogenic responsiveness develops as a result of maturation of neural pathways integrating the subfornical organ (sSFO) and organum vasculosum of the lamina terminalis (OVLT) with the median preoptic nucleus (MnPO). b) The development of endocrine modulation of dipsogenic responses coincides with the onset of specific receptor binding in the SFO, OVLT, and MnPO. And c) Fetal dipsogenic responses to hyperosmolarity may be fundamentally different from the adult, as a result of the developmental pattern of dipsogenic mechanisms. A stepwise series of experiments is proposed: 1) The ontogeny of the responsiveness of putative osmotic dipsogenic central nuclei (SFO, OVLT, MnPO) will be examined with neuronal activation (cFos) staining and nuclei miroinjections; 2) The essential role of these nuclei will be investigated via nuclei microablation; 3) The development of interconnecting dipsogenic neuronal pathways will be investigated with neural tract tracing techniques; and 4) The maturation of the endocrine modulation of dipsogenic responses will be correlated with changes in cerebral endocrine receptor binding and immunochemical staining.